1. Field of the Invention
The present invention relates, in general, to organic electroluminescent devices. More particularly, the present invention concerns an architecture for fabricating lighting panels including organic electroluminescent devices that provides current limitation in the event of a short.
2. Related Technology
Organic (which here includes organometallic) electroluminescent devices may be fabricated using polymers and/or small molecules in a range of colors, depending upon the materials used. Examples of polymer-based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of small molecule based devices are described in U.S. Pat. No. 4,539,507 and examples of dendrimer-based materials are described in WO 99/21935 and WO 02/067343.
A basic structure 100 of a typical organic electroluminescent device is shown in FIG. 1. A glass or plastic substrate 102 supports a transparent anode layer 104 comprising, for example, indium tin oxide (ITO) on which is deposited a number of functional layers such as a hole transport layer 106 and an organic electroluminescent layer 108. A cathode 110 is deposited over the organic electroluminescent layer 108. The functional layers may be deposited by, for example, spin coating (afterwards removing material from unwanted areas by plasma etching or laser ablation) or by selective deposition such as inkjet printing. In this latter case banks 112 may be formed on the substrate, for example using photoresist, to define wells into which the organic layers may be deposited. Such wells define light emitting areas or pixels of the display.
Cathode layer 110 typically comprises a low workfunction layer immediately adjacent electroluminescent layer 108, such as a low work function metal such as calcium for improved electron energy level matching, and a high conductivity layer of a metal or alloy (such as for example aluminum). Additional sub-layers may also be included in the cathode layer, for example silver.
In the example shown in FIG. 1, to drive the device a voltage is applied across the anode 104 and cathode 110 represented by battery 118. Light 120 is emitted through transparent anode 104 and substrate 102 and such devices are referred to as “bottom emitters”. Devices which emit through the cathode may also be constructed, for example, by keeping the thickness of cathode layer 110 less than around 50-100 nm so that the cathode is substantially transparent.
Organic light emitting devices have attracted increasing interest in recent years as lighting elements. In particular, in addition to lighting panels, such devices using commonly using white light emitting materials have potential applications to full color displays with the help of color filters, in backlights for liquid crystal displays (LCDs) and in solid-state lighting (SSL) devices.
An electroluminescent device as described in FIG. 1 and used as a lighting panel is preferably large area, but also extremely thin. A thin device of an order 100 to 200 nm thickness can have a very high capacitance causing a large charge to be stored during its operation.
The consequences of a short can be two-fold ranging from a loss of operation of the entire lighting panel to a discharge of charge through the short. The discharge of charge through the short can damage an individual pixel causing it to fail or emit light with different characteristics to the remaining pixels of the device meaning that the overall device produces a non-linear display.
Known solutions to preventing or at least mitigating shorts include a multi-stack device to make the electroluminescent device much thicker by additional layers or additional thickness of functional layers between and an anode and a cathode and so reduce the probability of shorts and the capacitance. Such an approach has a disadvantage of additional functional material and additional processing steps.
A further known solution is an approach described in WO/015600 in which an electroluminescent element is divided into sub-elements each with an individual fuse element.
According to WO/015600 an electroluminescent lighting element comprises four sub-elements in electrically parallel-arranged branches forming a pixel and, in each branch, an additional fuse element between one of the connections and a part of the layer of electroluminescent material. When a short circuit occurs between an anode and a cathode of one sub-element of a pixel, an associated fuse element is blown by a current peak in an associated current path. Accordingly the current is now spread across the remaining three sub-elements which continue to emit light so that the pixel continues to emit light.
WO/015600 requires additional fuse elements to be provided in any fabrication step, which has the disadvantage of increasing processing times and fabrication steps, such additional steps which can increase the likelihood of a short. Moreover, in the event of a short one sub-element is removed from the luminescence of a pixel and depending upon the area of each sub-element can cause the overall pixel to display large non-linear luminescence characteristics.